Medium armor requires the Medium Armor Proficiency feat. These include types such as scalemail and breastplate armors. Some armors are categorized as medium even though they are plate or half plate.
Heavy armor and Medium armor give the highest Magical Resistance Rating with no cap enforced.For a list of Heavy Armors available in the game, head on over the Medium Armor Category.

Robes do not require any feat. They are common for spellcasting classes as they don't have any proficiencies or spell failure.
Robes cap the Magical Resistance Rating at 50.
For a list of Robes available in the game, head on over the Robe Category.

Outfits do not require any feat. They are common for Monks, as they can be worn while centered.
Outfits cap the Magical Resistance Rating at 50.
For a list of Outfits available in the game, head on over the Outfit Category.